Was it built from the beginning with joists that can support a live load, or was it only designed for storage? There IS a difference. What is the interior height? Code requires a 7' height to be considered livable space and you have to allow for room for insulation. What type of access does it currently have? You will have to take up room for the stairs both in the attic and on the floor below. Does it currently have any windows? Are they legal egress size? I already mentioned insulation, but it's really important, as is how you are planning to heat and cool the additional space. A mini split would be the easiest, but if the system in your home is old, you may want to have new calculations done, add some ducting, and replace the home's current HVAC with a unit that can handle all of the space.

If the home was built from the beginning to be convertible, and has a nice high pitch with around 9' interior height, then you could have an easy project. That's not the same thing as an inexpensive project. Just a less expensive project than a difficult to convert space would be. 50K is about average.